Presently, most over-the-road heavy vehicles are fuelled by gasoline or diesel fuel. Because both gasoline and diesel fuelled internal combustion engines generate a considerable amount of pollutants such as oxides of nitrogen (NOx) and particulate matter (PM), engine manufacturers will be required to make improvements to their engines in order to comply with the new government regulatory standards regarding lower emissions of combustion products.
For diesel-cycle engines one approach that shows a significant improvement in lowering emissions involves substituting some of the diesel fuel with gaseous fuels such as natural gas, pure methane, ethane, liquefied petroleum gas, lighter flammable hydrocarbon derivatives, hydrogen, and blends of such fuels. Gaseous fuels are generally defined herein as fuels that are gaseous at atmospheric pressure and zero degrees Celsius. Whereas liquid fuels such as diesel are injected at very high pressures in order to atomize the fuel, gaseous fuels can be injected into an engine's combustion chamber at lower pressure because no extra energy is required for fuel atomization.
An advantage of substituting a gaseous fuel for diesel fuel is that the selected gaseous fuel can be one that burns cleaner than diesel fuels while preserving the high efficiency and high torque of the conventional diesel engines.
Another advantage of gaseous fuels is that, as a resource, such fuels are more widely distributed around the world and the amount of proven reserves of natural gas is much greater, compared to proven oil reserves.
Gaseous fuels can also come from renewable sources such as vent gases from garbage dumps and sewage treatment plants. Hydrogen can be produced with electricity generated from renewable sources such as wind power and hydro-electric dams.
However, some modifications are required to a conventional diesel engine to allow gaseous fuels to be substituted for diesel fuel. In a diesel engine, the heat produced by the mechanical compression of the fuel and air mixture auto-ignites the liquid diesel fuel charge at or near the end of the piston's compression stroke. Under the same conditions, gaseous fuels such as natural gas will not reliably auto-ignite. Therefore, in order to burn a gaseous fuel in a conventional engine with the same compression ratio without having to completely redesign the engine, some additional device is required to assist with ignition of the gaseous fuel, such as a hot surface provided by a glow plug, or a fuel injection valve for introducing a pilot fuel. The pilot fuel can be a small quantity of diesel fuel, whereby the auto-ignition of the pilot fuel triggers the ignition of gaseous fuel.
Gaseous fuelled engines have to respond to different power and load requirements according to different conditions, either external to the engine system or internal to the engine system, while preserving or further reducing a vehicle's emissions rates. Examples of conditions that are external to the engine include ambient temperature, cold start and high or low transient loads depending upon where the vehicle travels, such as city or highway conditions or different altitudes. Examples of conditions internal to the engine include, predefined engine operating modes such as an engine protection mode or a particulate filter regeneration mode.
It is presently known in the diesel engine industry, and particularly in the heavy duty truck industry using diesel engines, to select engine fuelling strategies based on the geographic conditions, wherein the geographic conditions are either presumed from certain engine and/or vehicle operational parameters (for example, engine acceleration) or more accurately determined based on the vehicle location which is communicated through a receiver associated with the travelling vehicle as described in U.S. Pat. No. 5,983,156. Such a system comprises a control computer with a memory having a number of different engine fuelling maps stored therein. Each engine fuelling map includes a mathematical function, table of values or the like, mapping engine fuelling requests to appropriate fuel quantities, fuel injection timing, and the like. The control computer responds to the radio signals relating to the vehicle location to determine therefrom the vehicle's geographical location, retrieves from the memory unit an appropriate engine fuelling map and controls the fuelling system according to the appropriate engine fuelling map.
Due to the modifications to conventional diesel engines that are required for allowing gaseous fuels to be substituted for diesel fuel, a more sophisticated electronic engine control is necessary in order to implement new combustion strategies for gaseous fuelled engines. For example, the engine control strategies of a diesel engine that has been modified to operate with gaseous fuel has to take into consideration the timing for triggering the ignition assisted devices, while also coordinating such timing with the timing for the gaseous fuel injection to achieve efficient combustion.
While it has been demonstrated that by injecting a gaseous fuel directly into the combustion chamber of an internal combustion engine, it is possible to at least match the power output, performance, and efficiency of a conventional diesel engine, there are a number of factors that make gaseous-fuelled engines different from conventional diesel-fuelled engines, and these differences require control methods that are different from those developed for conventional diesel engines.